We are studying a post-transitional modification of tubulin involving the reversible enzymatic addition of a C-terminal tyrosine. We have identified ways the modification can modulate in vitro assembly: tyrosinolated tubulin bound more MAP-2, bound less phospho-MAP-2, assembled better with taxol, and formed shorter microtubules with MAP-2. In vivo, the apparent acceleration of tyrosinolation associated with leukocyte chemotaxis was shown not to be due to changes in the tyrosine pool; however in activated cells 2/3 of the tyrosine was replaced by an unknown catabolite. With HeLa cells we confirmed that tubulin tyrosinolated in vivo may be a structural variant from that which is a substrate in vitro. We have isolated a brain protein (unrelated to the tyrosinolation project) which specifically binds to and precipitates tubulin. It is an oligomer of a 50,000 dalton subunit distinct from any neurofilament or tau protein, and is uniquely resistant to phosphorylation. It might function to block indiscriminate assembly. It appears to bind less well in the presence of MAPS, but microtubules assembled in its presence were decorated with globular material.